Schistosomiasis (also known as bilharziasis, after Theodor Bilharz who identified the parasites) is a state of infection with leaf-like flat worms belonging to one or more species of blood flukes known as Schistosomatidae. Schistosomiasis is the most important among diseases caused by worms. Some 200 million people are infected by blood flukes (trematodes) in regions of Europe, Asia, South America, and also the Caribbean area. The disease complex results from infection by three major species of digenetic trematodes, viz., Schistosoma mansoni, S. japonicum, and S. haematobium. Fundamentally, an infected fresh water snail sheds free swimming infective forms (cercariae) into the water, and man (or other animal) is infected by the penetration of the parasite through the skin, followed by maturation of the worms (male and female) in the body, pairing of male and female worms, shedding of eggs in excrement into water where development occurs and host snails are then invaded for continuation of the cycle. In the mammalian host, the schistosomes enter blood circulation and pass through the lungs to mature in the liver, then reside in mesenteric-portal or pelvic veins. Eggs are shed by the female into the lumen of the small intestine in the case of S. japonicum, the colon (S. mansoni, and, rarely, S. haematobium) or the bladder (S. haematobium, or, rarely S. mansoni). Most of the pathological effects resulting from schistosome infections derive from the spined eggs, both within the body and in being shed in the urinary or fecal stream. Specific primary clinical problems occur in the intestine and bladder, together with secondary ones in liver, spleen, and lungs, plus variable involvement of the central nervous system and retina. The worms live for years (immune response of the host has little effort on established adult schistosomes, but does work against development of new infections. Pathological changes in schistosomiasis are considerably variable with the species and strain of parasite, duration of the infection, intercurrent infections, and nutritional state of the host.
Treatment of the schistosomiases does not reverse the damage already done the host by the parasitic worms. Anti-schistosomal agents generally impair the production of eggs and hinder development and functions of the flukes, with or without actually killing them. "Cure" is said to be achieved when viable eggs are no longer found in the excrement. Such criterion does not imply absence of worms, it must be understood. Successful treatment of the schistosomiases is difficult to achieve safely, for anti-schistosomal agents are appreciably toxic to the host. Suppressive management of schistosomiasis through administration of drugs at regular intervals may also be hazardous to the patient. Treatment of the infections is increasingly difficult in the sequence: S. haematobium, S. mansoni, and S. japonicum. That is essentially the same as the general extent of severity of the consequences of those schistosomiases.
Control of schistosomiasis through interruption of the life cycle of the parasite is a more attractive course of action than treatment of the infection. Two points at which control may be exercised include eradication of the snail intermediate host and prevention by protection of the mammalian final host against the cercariae shed by the snails. Various means have been tried to eliminate snails, for example, molluscicides and biological control; however, the basic problems have not been solved and even 0.2% of a snail population being infected renders a region highly endemic to schistosomiasis. Prevention of schistosomiasis, in sensu stricto, involves protection of man or other final host against infection by cercariae of the trematodes. In this regard, it would be desirable to have perdurable topical agents which, when applied to the skin, could afford means of safely preventing schistosomiasis. Hitherto, this goal has not been achieved.
It is known that various agents, when applied topically, provide some extent of protection of a final host against infection by penetration of the cercariae of Schistosoma mansoni or S. japoniucm. On the practical assessment of the results, however, the protective effects decrease markedly if the surface is exposed to washing or exposed to running water. Therefore, such topical agents offer little advantage in use by personnel (civilian or military) who may be exposed to waters containing schistosome-bearing snails. Practical utility of a topical anti-penetrant must include: resistance to washing action of flowing water, lack of irritant characteristics to the skin, ease of application, and low cost.